The Temperature Anisotropy and Helium Abundance Features of Alfvénic Slow Solar Wind Observed by Parker Solar Probe, Helios, and Wind Missions

Abstract

Slow solar wind is typically characterized as having low Alfvénicity, but the occasional occurrence of highly Alfvénic slow solar wind (HASSW) raises questions about its source regions and evolution. In this work, we conduct a statistical analysis of temperature anisotropy and helium abundance in HASSW using data from PSP within 0.25 AU, Helios between 0.3 AU and 1 AU, and Wind near 1 AU. Our findings reveal that HASSW is prevalent close to the Sun, with PSP observations displaying a distinct ``U-shaped" Alfvénicity distribution with respect to increasing solar wind speed, unlike the monotonic increase trend seen in Helios and Wind data. This highlights a previously unreported population of unusually low speed HASSW, which is found in both sub-Alfvénic and super-Alfvénic regimes. The observed decreasing overlap in temperature anisotropy between HASSW and fast solar wind (FSW) with increasing heliocentric distance suggests different underlying heating processes. Additionally, HASSW exhibits two distinct helium abundance populations, particularly evident in PSP data, with generally higher helium abundance compared to less Alfvénic slow solar wind. Moreover, the decreasing overlap in temperature anisotropy versus helium abundance distributions between HASSW and FSW with decreasing radial distance implies that not all HASSW originates from the same source region as FSW.

Accepted for publication

References in corpus (16)

• The Evolution and Role of Solar Wind Turbulence in the Inner Heliosphere
• The Solar Probe Cup on Parker Solar Probe
• First in-situ Measurements of Electron Density and Temperature from Quasi-Thermal Noise Spectroscopy with Parker Solar Probe/FIELDS
• Proton Temperature Anisotropy Variations in Inner Heliosphere Estimated with First Parker Solar Probe Observations
• Cross Helicity Reversals In Magnetic Switchbacks
• Sub-Alfvenic Solar Wind observed by PSP: Characterization of Turbulence, Anisotropy, Intermittency, and Switchback
• Intermittency in the Expanding Solar Wind: Observations from Parker Solar Probe (0.16au), Helios 1 (0.3-1au), and Voyager 1 (1-10au)
• Acceleration of polytropic solar wind: Parker Solar Probe observation and one-dimensional model
• Quantifying the Energy Budget in the Solar Wind from 13.3-100 Solar Radii
• Solar wind energy flux observations in the inner heliosphere: First results from Parker Solar Probe
• An Extended and Fragmented Alfvén Zone in the Young Solar Wind
• The Structure and Origin of Switchbacks: Parker Solar Probe Observations
• Parker Solar Probe Observations of High Plasma Beta Solar Wind from Streamer Belt
• Compositional metrics of fast and slow Alfvenic solar wind emerging from coronal holes and their boundaries
• Cross Helicity and the Helium Abundance as an in situ Metric of Solar Wind Acceleration
• The Temperature, Electron, and Pressure Characteristics of Switchbacks: Parker Solar Probe Observations

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